Exhausts from multi-cylindrical internal combustion engines are usually received in a manifold. A manifold comprises several branch lines that receive exhausts from the internal combustion engine's cylinders and a riser that receives the exhausts from the respective branch lines. Each cylinder generally comprises two exhaust valves. When the exhaust valves open, exhausts flow out into the connecting branch line with a high pressure, which is substantially related to the pressure of the exhausts in the cylinder right after the combustion stroke has ended. The pressure of the exhausts in the branch line during the remaining time, during which the exhaust valve is open, is lower and substantially related to the work of the piston in the cylinder when it presses the exhausts out into the branch line. The exhaust valves in the cylinders are normally open during the entire exhaust stroke, i.e. during a relatively large part of a four stroke engine's working cycle. The more cylinders in an internal combustion engine that are connected to a manifold, the harder it is to prevent the exhaust valves' opening times of several cylinders from overlapping at some time during the working cycle. In a manifold receiving exhausts from four cylinders, it is substantially impossible to create a firing order, such that the inlet opening times of the exhaust valves do not overlap each other at some point. On such occasions, exhausts are thus led out into the riser from several cylinders simultaneously.
It is not uncomplicated to lead exhausts out from several cylinders simultaneously into a riser. When a cylinder opens at the same time as exhausts are led out of another cylinder with a lower pressure, there is an obvious risk that the exhausts with the higher pressure penetrate down into the branch line ejecting exhausts with the lower pressure. Thus, the pressure in this branch line increases and the piston in this cylinder must work harder to eject the exhausts. The increased ejection work results in an increased fuel consumption of the internal combustion engine.
U.S. Pat. No. 5,860,278 shows a riser for receipt of exhausts from an internal combustion engine via a number of branch lines. The riser comprises constrictions in connection with all the outlets of the branch lines. Thus, the exhausts in the riser obtain an increased speed and a reduced static pressure in connection with the outlets in the riser. Accordingly, exhausts with a lower pressure may be ejected into the riser. However, the adaptation of the riser with constrictions at all outlets has the disadvantage of relatively large exhaust flow losses in the riser.